Field of the Invention
The present invention relates to measurement of temperatures and distances at remote locations followed by transmitting the results, and more particularly, to systems transmitting such measured information in the form of relative timings of electrical pulses.
Many situations arise where information is measured at one location and then the results thereof are transmitted to another location for the convenience of the user. The usual desire to reduce material cost and system complexity makes advantageous the use of the simplest and most inexpensive means for transmitting such information permitted by the circumstances. This typically means the use of some sort of electrical cabling. Cost and complexity will normally then be minimized by choosing cabling having as few conductors as possible for a suitable type of cable.
A common situation requiring measurements of this sort is the desire to measure the volume of the contents of a tank and the temperature or other parameter of such contents, this information to be reported to a central data collection area for record keeping or for control purposes, or both. The liquid contents of a tank of fixed size can be measured by noting the distance of the liquid surface from the top of the tank.
A basis for such a measurement is the Wiedemann Effect where an electrical conductor conducting an electrical current pulse therealong will experience a twisting effect at the point that the magnetic field induced by the pulse interacts with any other magnetic field encountered along its path. This twisting effect will lead to a torsional sonic pulse transmitted along the same conductor. A better result is obtained if the electrical conductor is a thin wire mounted inside a thin-walled, ferromagnetic material tube, the wire and tube together forming a sonic waveguide.
If such a waveguide is placed vertically in the tank extending down to near the bottom thereof, and if a float containing a magnet is allowed to move along the sonic waveguide in response to the level of liquid in the tank, the distance of the top of the liquid to the top of the tank can be measured. An electrical pulse sent down the electrical conductor in the sonic waveguide, having a magnetic field associated therewith, will, upon reaching the float, interact with the magnetic field provided by the magnet in the float. At that point, a torsional sonic pulse will be transmitted along the tube which will be conveyed back to the operating circuitry associated with the waveguide at a reference position thereon at or near the top of the tank.
A mode converter is provided on that part of the sonic waveguide tube occurring at the reference location which is formed as a pair of flat straps, or tapes, fixed on opposite sides of the tube. Each of these tapes is formed of a magnetostrictive material. A torsional sonic pulse along the tube reaches the tapes and momentarily expands one and simultaneously momentarily contracts the other tape. This pulsed contraction and expansion of the tapes causes a magnetic change in wiring coils surrounding each of the tapes to thereby provide an electrical signal across these coils. The coils are interconnected so that the signals in each add to provide a signal of suitable size to indicate the arrival of the sonic torsional pulse at the tapes. A permanent magnet provides an initial magnetic bias for such coils.
Thus, the time duration from the start of the electrical pulse down the sonic waveguide wire conductor until the return of the sonic pulse on the waveguide tube (which is transformed into an electrical pulse by the mode converter) is a measure of the distance to the magnet in the float because of the known propagation velocity of such pulses. Hence, the distance is measured to the surface of the liquid. The propagation time of the electrical pulse down the electrical conductor is negligable compared to the propagation velocity of the sonic torsional pulse along the tube. The sonic torsional pulse propagation velocity will be determined by the modulus elasticity of the tube which can be made independent of temperature for certain choices of material.
Sensing the temperature of the liquid in a tank can be readily accomplished by the use of either temperature sensitive resistors or of temperature sensitive semiconductor devices. Such sensors typically provide a voltage thereacross which is indicative of the temperature of the region in which such devices are located. For most tank situations, for example, there will be a desire to know the temperature at several locations in the tank and, therefore, several sensors would be spotted about the tank at those locations.
The results measured concerning the liquid contents of a tank in this example must then be conveyed to the user, typically at some centralized location. Therefore, a system for operating these kinds of sensors and for conveying the results of such measurements along a simple and cost effective transmission path is desirable.